Neutrino History — Part 2: The first detection, the muon neutrino, Davis begins (1956–1970)

This is Part 2 of the Neutrino History series. In Part 1 we left the field in 1955 with the neutrino still theoretical. In Part 2 we watch the field transform: the particle gets caught, multiple flavors are discovered, and the solar neutrino problem begins its 33-year run.

1956: The Savannah River detection

Frederick Reines and Clyde Cowan had been working on neutrino detection since 1951. Their original proposal, half-seriously, was to detonate a nuclear bomb in the desert and put a detector down a deep mine shaft nearby. Cooler heads pointed out that nuclear reactors would work just as well.

They built a sandwich detector: two tanks of liquid scintillator surrounding a water tank containing dissolved cadmium. An antineutrino from the reactor would occasionally hit a proton, producing a positron (prompt flash) and a neutron (delayed flash after cadmium capture). The double-flash signature with the right timing was unmistakable.

In June 1956, after several years of patient running at the Savannah River nuclear reactor in South Carolina, the team had enough events to declare detection. They sent a telegram to Pauli at the Swiss Federal Institute of Technology:

We are happy to inform you that we have definitely detected neutrinos from fission fragments by observing inverse beta decay of protons. Observed cross-section agrees well with expected six times ten to the minus forty-four square centimeters.

Pauli reportedly received the telegram during a conference and opened champagne with his colleagues. The “desperate remedy” of 1930 was now a confirmed particle. Reines would win the Nobel Prize 39 years later, in 1995. Cowan died in 1974, before the recognition came.

1957: Parity violation

In late 1956, Tsung-Dao Lee and Chen-Ning Yang proposed that the weak interaction might violate parity — meaning the laws of physics could distinguish left from right at the most fundamental level. The proposal was theoretically motivated by puzzles in kaon decay but needed direct experimental test.

Chien-Shiung Wu at Columbia University took on the test. In early 1957, she led the experiment that cooled cobalt-60 nuclei to about 0.01 K, aligned their spins with a strong magnetic field, and measured the angular distribution of the beta-decay electrons. The electrons came out preferentially against the direction of the nuclear spin — a clear, unmistakable asymmetry.

The Wu experiment proved that the weak interaction violates parity. The implication for neutrinos was immediate: they are produced only in left-handed helicity states, and antineutrinos only in right-handed states. This “left-handedness” became one of the foundational features of the Standard Model.

The 1957 Nobel Prize in Physics went to Lee and Yang for the theoretical proposal. Wu, who performed the decisive experiment, was famously not included.

1957–1958: Pontecorvo’s oscillation idea

While Wu was running her experiment in Washington, Bruno Pontecorvo at the Joint Institute for Nuclear Research in Dubna, Soviet Union, was thinking about something even more unusual. He wondered: if there are multiple kinds of neutrino, can one kind transform into another in flight?

In 1957 he published “Mesonium and antimesonium,” considering neutrino-antineutrino oscillation by analogy with the well-known K⁰-K̄⁰ system. In 1958 he extended the framework. The full three-flavor picture was developed in 1962 with Maki, Nakagawa, and Sakata. The resulting framework — neutrinos as mixtures of mass states, oscillating between flavors in flight — is now called the PMNS matrix and was confirmed experimentally only in 1998.

Pontecorvo’s 1957 prediction was 41 years ahead of its experimental confirmation.

1962: The muon neutrino

By the late 1950s, physicists had begun to suspect that the neutrinos involved in muon physics might be a different particle from the neutrinos in electron physics. The hint came from how certain rare processes did or didn’t happen.

Leon Lederman, Melvin Schwartz, and Jack Steinberger tested it at Brookhaven National Laboratory. They fired pions at high speed, let them decay (mostly into muons and muon-neutrinos), and shielded the beam with 13.4 meters of steel armor plate to block everything except the neutrinos. A spark chamber detector then waited for interactions.

The result was clear: when the muon neutrinos interacted, they produced muons — never electrons. They had to be a distinct kind of neutrino. The 1962 experiment confirmed the muon neutrino as separate from the electron neutrino, establishing the lepton-flavor structure that became part of the Standard Model. Lederman, Schwartz, and Steinberger shared the 1988 Nobel Prize.

A third flavor — the tau neutrino — was inferred theoretically after Martin Perl discovered the tau lepton in 1975. It was directly observed only in 2000 by the DONUT experiment.

1968: Davis at Homestake

Twenty-two years after Pontecorvo’s chlorine proposal, Raymond Davis Jr. began running the experiment that would test it. He installed a 600-ton tank of perchloroethylene (a common dry-cleaning fluid) at the Homestake gold mine in Lead, South Dakota — 1,478 meters underground, where cosmic-ray backgrounds were sufficiently shielded.

Davis’s plan was to count the very few argon-37 atoms produced when solar electron neutrinos interacted with chlorine-37 in the tank. The expected rate, based on John Bahcall’s standard solar model, was about 8 events per day.

The actual rate, as Davis began publishing in the early 1970s, was about a third of that. The deficit persisted year after year. The solar neutrino problem had begun.

For thirty years, the field would argue about whether the solar model was wrong or the neutrinos were doing something unexpected on their trip from the Sun. The answer turned out to be the latter — but the experiments needed to prove it would take three decades to build.

1970: The end of an era

By 1970, the basic structure of the neutrino sector was in place. The particle was real. Parity violation was confirmed. Two flavors were known, with a third predicted. The solar neutrino problem was emerging. And Pontecorvo’s oscillation hypothesis was sitting in the literature, waiting to be tested.

The field was now poised for the long experimental campaign of the 1970s, 1980s, and 1990s — patiently building larger and larger detectors that would eventually confirm oscillation, resolve the solar problem, and catch the first supernova neutrinos. In Part 3, we’ll watch that campaign build through to SN 1987A.

Neutrino Times Editorial Team. (2026, January 31). Neutrino History — Part 2: The first detection, the muon neutrino, Davis begins (1956–1970). Neutrino Times. https://neutrino-times.com/articles/neutrino-history-part-2-1956-1970/

Neutrino Times Editorial Team. "Neutrino History — Part 2: The first detection, the muon neutrino, Davis begins (1956–1970)." Neutrino Times, January 31, 2026. https://neutrino-times.com/articles/neutrino-history-part-2-1956-1970/.

Neutrino Times Editorial Team. "Neutrino History — Part 2: The first detection, the muon neutrino, Davis begins (1956–1970)." Neutrino Times, 31 Jan. 2026, https://neutrino-times.com/articles/neutrino-history-part-2-1956-1970/.

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